What people who speak of three basic "vocations" really mean is three "states of life," which is not quite the same thing and shouldn't be spoken of as if it were. More clarity is needed, if only to aid people's discernment and sense of Christian identity. Now I'm sure there's good writing about this topic out there somewhere. I welcome readers' suggestions for that. In the meantime, I tentatively offer a few thoughts that have been stimulated by meditation and discussion, but not by any systematic reading on the topic.
All Christians have the same vocation: the baptismal vocation. In baptism, we die to the old man and rise to new life in Christ. Thus each Christian, even those baptized as infants, become part of
"a chosen race, a royal priesthood, a holy nation, a people of his own, so that you may announce the praises" of him who called you out of darkness into his wonderful light. Once you were "no people" but now you are God's people; you "had not received mercy" but now you have received mercy (1 Peter 2: 9-10; emphasis added).Each of us shares in the priesthood of believers. For those who live to the age of reason, that is manifest when our faith is intentional enough to sustain personal prayer and sacrifice. Those are always necessary for answering "the universal call to holiness" (Lumen Gentium, Ch. V). The ministerial priesthood exists to facilitate and serve that priesthood of believers in certain prescribed ways. Thus the ministerial priesthood, like the life of the non-ordained "religious," is for the Church what the Church is for the world at large. But whether laity or clergy, we all are "called" out of the world to be what Peter says. We all have that vocation.
Marriage and consecrated life are distinct states of life, though they can be combined by married clergy. Yet I dislike speaking of them as two distinct vocations. Both are forms of living that love to which all Christians are called by virtue of the baptismal vocation. They are two modes of living out the one, baptismal vocation.
But the latter is a clearer mode than the former. If my saying that seems strange, it shouldn't. Until the 20th century, Catholic theologians were reluctant to speak of marriage as a vocation at all. I don't know about the Orthodox, but I suspect the same is true for them. There was a very good reason for that. Most people marry and have children at some point, which is what God intended from the beginning. To speak of marriage as a "vocation" in the strict sense of the term, i.e. as a way of being "called" out of something, suggests that marriage is a special state distinguishing the married from the mass of humanity. But it isn't, really. It's just something most people do at some point, whether they're Christian or not. For those who marry with the right intentions and capacities, of course, marriage is sacramental. But that doesn't make it any more special than just being a good Christian.
Admittedly, given the general breakdown of marriage today, along with people's weakening sense of what sacramental marriage is, it's become more appropriate to speak of the "vocation" of marriage than in the past. To be faithfully married as the Church understands marriage is a noble thing indeed, considering what marriage is in the world today even among Catholics. So there is a sense in which marriage can be well spoken of as a vocation. At the same time, it should be remembered that marriage is not, objectively speaking, as clear a witness to the Gospel as consecrated life involving celibacy.
Marriage can be understood, appreciated, and contracted in purely secular terms. Though that approach to marriage is incomplete from the believer's standpoint, it cannot really be said that married unbelievers aren't married. But leaving aside physical and/or psychological impediments to marriage, a voluntary commitment to lifelong celibacy makes sense only in evangelical and eschatological terms. Hence, as most Catholics used to admit, it's not as appropriate to call marriage a "vocation" as it is to call the clerical state or religious life "vocations." Unlike the married, consecrated celibates really are "called" out of the normal human way of life. To be sure, the way some people live marriage is so exemplary that one can clearly see their marriages as beautiful expressions of the baptismal vocation. Some married folk just are better, holier witnesses to the Gospel than some clergy or religious. But that is more a matter of subjective intentionality than of the objective state itself. We don't expect the married to be holier than the average Catholic, the way we rightly expect clergy and religious to be, even when they aren't. Such an asymmetry of expectation is not a hangover from the bad old days of clericalism. It corresponds to the objective reality of the respective states of life.
What about singles? There are two extremes to avoid here. One I've already rejected: thinking of lay singlehood as a formal, ecclesially recognizable "vocation" like consecrated life or, in a secondary sense, marriage. It isn't. For one thing, we all start out as lay singles. For another, some singles really are called to marriage or consecrated life but don't seem to be attaining either, usually because of their own or others' failings. But let's avoid the other extreme of thinking that all singles are called to marriage or consecrated life and are just failing to answer. Some lay singles must remain such because of impediments to marriage and consecrated life. Others have no obvious impediments, but have been given a special mission by God that doesn't fit into the two usual modes of living out the baptismal vocation. Those two categories of singles are of great significance for a theology of vocation.
The singles with "impediments," including but not limited to the mentally ill and the developmentally disabled, are those in whom Christ lives in his vulnerability, in how he takes on the tragedy of the human condition. Through them, he beckons us to love him under that aspect. To the extent we treat such people, "the least of my brethren," as Christ himself, they will show us his love in a special way that is easily lost in the hurly-burly of "normal" human life. They tell us that God loves us primarily for who we are, not for what we do. So it is good that most such people cannot "justify their existence" in any other way. None of us can justify our existence by what we do, for all is gift, "all is grace." The usual classes of singles with impediments remind us of that, and they should evoke our love accordingly.
The singles without impediments, who nevertheless have been given special missions that don't "fit in" with the usual ways of living out the baptismal vocation, remind us that the transforming activity of the Holy Spirit—otherwise known as grace—is not limited to anything that can be institutionalized, including the visible Church herself. That is a reality which the conventionally pious often forget or fail to appreciate. I'm thinking of people like the spinster sister who never left home because she was the one who ended up caring for the aged parents. Or the dedicated scholar who doesn't seem to have time for much of anything except her subject. Or the career soldier who'd like to marry but ends up sacrificing himself on a vital mission. Or the man who doesn't mind celibacy, but was too straight and Catholic to get through his diocese's seminary and instead devotes his life to lay ministry, perhaps as a missionary. There are many people like such singles. They testify that the baptismal vocation can and ought to be lived in any and all circumstances, not just the usual states of life.
If we're going to speak of a vocation to singlehood, then, what we should mean is a vocation to live the baptismal vocation merely as such. But that is just to say that the baptismal vocation is what's fundamental, not the formal mode by which it is lived. Maybe God wills singlehood for some in the Church primarily to remind the rest of the Church that the mode in which we live said vocation is less important than the generosity with which we live it. That is a peculiar challenge for singles themselves, who often have no obvious human commitments to evoke their generosity. But it is, after all, is what the universal call to holiness assumes.